Even as moralists and activists continue to debate homosexuality, many species casually practice it

Giraffes do it, goats do it, birds and bonobos and dolphins do it. Humans beings--a lot of them anyway--like to do it too, but of all the planet's species, they're the only ones who are oppressed when they try.

What humans share with so many other animals, it now appears, is freewheeling homosexuality. For centuries opponents of gay rights have seen same-gender sex as a uniquely human phenomenon, one of the many ways our famously corruptible species flouts the laws of nature. But nature's morality, it seems, may be remarkably flexible, at least if the new book Biological Exuberance (St. Martin's...